1. Field of Invention
The present invention relates to a conversion of sound to electrical signals in either analog or digital form. Sound as detected by microphones is caused by pressure changes in a medium.
2. Description of the Related Art
Sound is a propagating change or series of changes in the local pressure of a medium. The device which converts these pressure changes into electrical signals is the microphone. The microphone generally consists of some sort of transduction element, which physically moves in response to the pressure changes, and a mechanism which converts this physical motion into an electrical output signal. In most cases the transduction element is a diaphragm, but this is not always the case. In the current art, there are several commonly used detection mechanisms. In what follows, the description will focus on microphones designed for use in picking up audible sound in air, however the general principal of the present invention may be applied to all other microphone types.
There are disadvantages to this type of system. Firstly, the diaphragm (or other transduction element) is supported by a spring composed of the tension of the diaphragm itself, among other supports which depend upon microphone type. This spring will be imperfect, and will be acceptably linear only up to a maximum sound pressure level. Secondly, the diaphragm will have mass, which will limit the frequencies to which the microphone may respond, and which will introduce one or more resonant frequencies, which will result in non-linearity of frequency response at and near a given resonant frequency. Further, there is always an analog electronic system interfacing the diaphragm with the balance of system. This interface will itself be frequency sensitive and subject to non-linearity.
Realized microphone designs strive to minimize the effect of these various distortions in the intensity and frequency range of interest, through suitable selection of diaphragm type and electronic components. However, highly linear microphones are expensive and complex, and as the present invention will show, highly linear microphone systems can be constructed with more accurate output as well as lower cost. Presently, this is not available to the art.
Feedback is the principal of returning the output of an amplified system to the input, so as to reduce the gain of the system. While a reduction in gain is often considered a detriment, the benefits are that the linearity of the amplification system becomes dominated by the linearity of the feedback device.
While it is quite difficult to produce a linear amplifier, it is quite easy to produce an extremely high gain but non-linear amplifier. Coupled to highly linear resistive feedback, such an amplifier can be used to construct a highly linear, moderate gain system.
The analog to digital converter is a device which converts an input analog signal into a digital representation. This digital representation consists of a series of numbers which represent the amplitude of the analog input at specific moments in time. The series of numbers is a discretely sampled, quantized representation of the input.
Of particular interest are differential coding techniques, in which the output of an initial conversion step is subtracted from the input for the next conversion step, creating an output which corresponds to the difference between one sample and the next. With the addition of an integration filter, sigma-differential coding can be achieved, wherein the output of the converter is again a direct representation of the analog input, however the spectrum of the quantization noise is shifted to higher frequencies.
The limit of this design paradigm is the one bit sigma-differential pulse code modulator, or "Sigma-Delta" converter. With reference to FIG. 4, the quantized output of the one bit digital to analog converter is subtracted from the input analog signal, with the difference being integrated. The output of the integrator is fed to a comparator which acts as a single bit analog to digital converter. The output of this converter both supplies the digital output to the rest of the system, as well as providing the quantized output to be subtracted from the input.
The operation of the sigma-delta converter is as follows. The output of the integrator is evaluated by the comparator. The comparator outputs a "1" if the integrator output is above the reference value, and a "0" if the integrator output is below the reference value. The analog values of "1" and "0" are such that, through the differential stage feedback, a "1" will tend to cause the integrator output to fall below the reference value, and a "0" will tend to cause the integrator output to climb above the reference value. Over time, the duty cycle of "1" and "0" will represent the value of the input signal. Often higher order feedback loops are used which will tend to decrease the low frequency quantization noise at the expense of high frequency noise.
In common audio use is a sigma-delta converter with a sampling rate equal to sixty-four times the desired decimated output, using 5th order feedback in the sigma-delta conversion stage. The one bit output of this converter is decimated using digital techniques, and produces a 16 bit output with performance of a 16 bit linear converter.
Sigma-Differential conversion techniques, and the sigma-delta converter may be considered examples of the digital use of negative feedback in the digital domain. The quantizer is a highly non-linear device, however through the use of feedback substantial linearity may be achieved.
The method of the present invention makes new use of the concept of sigma-differential conversion techniques.